Prime Minister John Key says technical support has been dispatched to Havelock North to find out the cause, and an inquiry is underway.

"It's far too early to point fingers, but we have to find out what took place, what contaminated the water and what the responses were," he says

"There will be an inquiry… It probably doesn't make sense to have an independent inquiry running alongside the inquiry that's going to be run by the council independently - I think it'll probably make sense to be combined."

"This is a couple of thousand people, and people seriously ill in hospital. It's not acceptable so we have to work out what's caused that and why."

More than 1000 people are believed to have been hit by the stomach bug and schools in the Hastings suburb are closed.

Mr Yule on Monday described the scale of the outbreak as unprecedented.

The test results could be out on Tuesday, it's been reported.

The illness has left two people in a critical condition in Hawke's Bay Hospital.

On Monday, the council published a full-page apology in Hawkes Bay Today, signed by Mr Yule and chief executive Ross McLeod.